Upgrading hypervisors typically involves rebooting the underlying machine from the pre-upgrade version to the post update version. Any virtual machines running on the hypervisor are interrupted during the rebooting. To minimize such interruptions, the virtual machines can be migrated to a standby machine (e.g., using vMotion, available from VMware, Inc.) so that the virtual machines can continue executing during the upgrade. If the standby machine is running the upgraded version of the hypervisor, then the migrated virtual machines can remain there. Otherwise, the migrated virtual machines can be returned to the original machine once it is upgraded. However, if the collective size of the virtual machines is large, the migration(s) can take an undesirably long time. Furthermore, the required standby machine is not always available. Similar problems apply to upgrading of operating systems other than hypervisors.